Heretofore, an anisotropic etching technique has been used to form bodies of silicon using the property that certain crystallographic planes of a semiconductor material will etch differently than other planes. For example, for single crystalline silicon, etching takes place slower along the &lt;111&gt; planes than along the &lt;100&gt; and &lt;110&gt; planes in etchants with sufficiently high pH values. Typical etchants include KOH, NaOH, LiOH, CsOH, NH.sub.4 OH, ethylenediamine pyrocatechol, and hydrazine. In addition to the crystal orientation selectivity of these etchants, a sufficiently high constant positive bias applied between a silicon body and electrode both immersed in the etchant will also passivate the surface of the body and stop the etching. This phenomena is referred to as "electrochemical etch stop." The bias required to passivate the surface of the silicon body is denoted as the "passivating potential (voltage)."
The electrochemical etch stop has been utilized in forming the bodies of silicon by starting with a body having a region of n-type conductivity adjacent a region of p-type conductivity with a p-n junction therebetween. The n-type region is formed of the thickness desired for the thin body. The starting body is placed in the chemical etchant with the surface of the p-type region being exposed for etching, and a constant positive bias at or more positive than the passivating potential is applied between the starting body and an electrode in the chemical etchant and spaced from the starting body. This positive voltage reverse-biases the p-n junction so that no current flows into the p-type region. The p-type region is anisotropically etched away by the chemical etchant until it is completely removed. When the p-type region is completely removed so as to expose the n-type region, the positive voltage between the n-type region and the electrode causes a passivation layer of silicon oxide to be formed over the surface of the n-type region which stops the chemical etching. This leaves a thin silicon body of the desired thickness.
A problem with this type of electrochemical etching is that a constant positive bias can cause indiscriminate etch stopping on both p-type and n-type silicon if current flows through the silicon body and into the etchant. Therefore, it is necessary to have a reverse biased junction in order to prevent the flow of current through the p-type region and selectively remove the p-type region while stopping only when the n-type region is reached. However, if there are large reverse leakage currents passing through the junction, the p-type region will be prematurely passivated causing the etching to stop before all of the p-type region is removed. Therefore, it would be desirable to have an electrochemical etching technique which does not rely on the reverse biased junction to control the etching. Also, this technique removes the p-type material leaving the n-type material, so that it cannot be used in cases where a thin layer of p-type material is desired.